Which disease is most confused with smallpox?

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When evaluating a vesicular rash that could be smallpox, the key idea is how the rash develops and looks. Chickenpox is most often mistaken for smallpox because its rash can resemble smallpox early on: fever and vesicular lesions appear, and lesions may be seen in crops. Varicella lesions are typically superficial and appear in different stages at the same time, with a trunk-heavy distribution. In contrast, smallpox produces deeper lesions that tend to evolve in a synchronized fashion across the body and often involve the face and extremities, including the palms and soles. These pattern differences help distinguish the two, so chickenpox is the most common source of confusion with smallpox. Measles lacks vesicular lesions and presents with a maculopapular rash plus cough, coryza, and conjunctivitis; monkeypox can resemble smallpox but usually has notable lymphadenopathy and specific exposure history, making it easier to differentiate in many cases.